Final answer:
The temperature light in a vehicle will switch on when the engine's coolant becomes too hot, indicating an issue with the vehicle's cooling system.
Step-by-step explanation:
The temperature light will come on in your vehicle when the coolant in the engine is too hot. This is a warning signal that indicates the engine temperature has exceeded the normal operating range, which could be due to a variety of reasons such as low coolant levels, a malfunctioning radiator, or a problem with the water pump. This system is critical for preventing engine overheating and potential damage.